staging: vt6656: desc.h remove typedef struct tagSTxBufHead to rxtx.h

In code it is referred to as fifo to renamed to
typedef struct tagSTxBufHead -> struct vnt_tx_fifo_head

This is only needed by rxtc.c so moved to rxtx.h

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>